In recent years, among multi-component oxides containing bismuth as a constituent element, Aurivillius layered Bi compounds (see Nonpatent Reference 1) have attracted attention for their good ferroelectric property. An Aurivillius layered Bi compound has its chemical composition expressed by formula: (Bi2O2)Am−1BmO3m+1 (where A is Sr, Ba, Ca or Bi, B is Ti, Ta or Nb and m≧1. Composition Bi4Ti3O12, which is where in the chemical composition formula, A is Bi, B is Ti sand m is 3, is especially excellent in ferroelectric property and its practical use as a FeRAM (Ferroelectric Random Access Memory) material is being investigated. Compound Bi4Ti3O12 is a material having its complex composition and structure in which three layers of TiO6 octahedron as a perovskite type slab structure and a Bi2O2 layer are alternately disposed. Consequently, to form a thin film of crystal perfection of a level such that it can be used as a FeRAM, the conventional epitaxy method makes it hard to achieve this goal and the present inventors have already proposed the flux epitaxy (see Nonpatent Reference 2 and Patent Reference 1). According to this method, it is possible to manufacture a Bi4Ti3O12 single crystal thin film having a crystal perfection to an extent that it justifies its use as a FeRAM.
Aurivillius layered Bi compounds are excellent not only in ferroelectric property but also in piezoelectric and pyroelectric properties. By the way, attention has been riveted to MEMS (Micro-electro-mechanical system) techniques. MEMS is a system in which a microelectronic integrated circuit, a microminiature sensor and an actuator are integrated on, for example, a substrate such as Si single crystal, and as it were a system that puts a brain, an eye and an arm together to integrate the miniaturization, energy saving and high reliability. It is used, for example, in airbag systems in automobiles or a printer for personal computer, becoming an indispensable technique in the current society. MEMSs, those are also indispensable techniques for high-functionalization of mobile phones, biochips and others, have vigorously been investigated and developed. The size required for such sensors and actuators is in the order of μm or even sub μm, and the materials that can fully exhibit their functions and the manufacturing techniques for forming those of this size are being searched for.
The conventional method of making microminiature sensors and actuators necessary in MEMSs is what is called top-down method. The top-down method is a process that is carving a micro-configured component from a bulk single crystal using a semiconductor photolithographic technique. This method, however, becomes extremely costly if the size of a component to be fabricated is down to sub μm order. That is, in the lithographic technique for sub μm or less, very-short ultraviolet ray or X ray must be the exposure light source and the cost for such an exposure apparatus is extremely high. Also, using electron beam for exposure makes the throughput low and the production cost high.
As a process to attempt to solve such problems, the so-called bottom-up method has drawn attention in recent years. The bottom-up method is a method of fabrication that utilizes the nature of atoms and molecules to spontaneously create an orderly structure by the properties they originally possess, i.e., the self-organization. As an example of the bottom-up method, there is a process of making Si nanodots (see Nonpatent Reference 3). In this method, Si nanodots of single crystal are spontaneously formed in the form of islands on a substrate by subjecting the substrate to LPCVD (low pressure chemical vapor deposition) with silane gas upon causing Si of SiO2 to be terminated with hydroxyl group by hydrofluoric-treatment, and such nanodots have been used as quantum dots for a single-electron transistor. It is also known that when a Bi4Ti3O12 thin film with excessive Bi is epitaxially grown on an LSCO (La0.5Sr0.5CoO3) substrate, Bi nanodots are spontaneously arrayed orderly on the Bi4Ti3O12 thin film (see Nonpatent Reference 5). It has been investigated if such nanodots can be utilized as FeRAM electrodes.
Reference cited:
Nonpatent Reference 1: J. F. Scott, translated jointly by Hitohiro Tanaka, Kaoru Miura, and Chiharu Isobe “Ferroelectric Memory (from its Physics to Applications)”, Springer-Fairlark Tokyo, 1st Ed, page 0.163;
Nonpatent Reference 2: Ryuta Takahashi et al., “High Temperature Superconduction by Tri-phase Epitaxy, Preparation of Single Crystal Thin Films” Journal of the Japan Institute of Metals, Vol. 66, No. 4 (2002), 284-288, Special Edition “Recent Superconducting Materials”;
Nonpatent Reference 3: S. Miyazaki et al., “Control of self-assembling formation of nanometer silicon dots by low pressure chemical deposition”, Thin Solid Films 369 (2000), 55-59;
Nonpatent Reference 4: M. Alexe et al., “Self-patterning nano-electrode on ferroelectric thin films for gigabit memory applications” APPLIED PHYSICS LETTERS Volume 73, Number 11, 14 September (1998), 1592-1594;
Nonpatent Reference 5: Wei F. Yao et al., “Synthesis and photocatalytic property of bismuth titanate Bi4Ti3O12” Material Letters 57 (2003), 1899-1902; and
Patent Reference 1: Japanese Patent Application No. 2004-85232